1.1.2. Hypervisor

If virtualization is defined as enabling multiple operating
systems to run on a single host computer, then the essential
component in the virtualization stack is the hypervisor. This
hypervisor, also called Virtual Machine Monitor (VMM), creates a
virtual platform on the host computer, on top of which multiple
guest operating systems are executed and monitored. This way,
multiple operating systems, which are either multiple instances
of the same operating system, or different operating systems,
can share the hardware resources offered by the host.

Native hypervisors are software systems that run
directly on the host's hardware to control the
hardware, and to monitor the guest operating systems.
Consequently, the guest operating system runs on a
separate level above the hypervisor. Examples of this
classic implementation of virtual machine architecture
are Oracle VM, Microsoft Hyper-V, VMWare ESX and Xen.

Type 2: hosted

Hosted hypervisors are designed to run within a
traditional operating system. In other words, a hosted
hypervisor adds a distinct software layer on top of
the host operating system, and the guest operating
system becomes a third software level above the
hardware. A well-known example of a hosted hypervisor
is Oracle VM VirtualBox. Others include VMWare Server
and Workstation, Microsoft Virtual PC, KVM, QEMU and
Parallels.